Semiconductor detectors, due to their higher detection efficiency and better energy resolution, are widely concerned, and applied to various applications of the radiation detection, for example, nuclide identification devices, alarming radiation dosimeter, etc. in environmental radiation detection; item detection devices, such as item inspection machines and industrial computer tomography (CT), in national security; CT, dental imaging, positron emission tomography (PET), single photon computer tomography (SPECT), etc. in medical applications. There are many types of semiconductor materials, such as CdMnTe (cadmium manganese telluride), HgI2 (Mercuric iodide), TlBr (Thallium bromide), PbI2 (Lead iodide), GaAs (Gallium arsenide), Ge (germanium), and so on, which are applied to different areas due to their different characteristics.
CdZnTe (cadmium zinc telluride, abbreviated to CZT), in addition to its good energy resolution and high detection efficiency, can work at the room temperature, which enables it to be the most promising material for radiation detection. With the CZT semiconductor, detectors designed in a pixel-based structure can be applied in a number of areas of radiation imaging, such as dental CT, SPECT and so on.
The pixel-based structure can obtain not only a good energy resolution but also a relatively high spatial resolution, and thus has wide application prospects in astronomical imaging, medical imaging, and other aspects.
A pixel electrode (pixel cathode or pixel anode) is a unipolar charge sensitive technique, with induced charges contributed by drifting of only one type of carriers. Unlike a uniform electric field in a planar detector, a pixel-based detector has a non-uniform electric field distribution therein. A generated free charge, when drifting in a region distant to pixel electrodes, induces very small charge on a single pixel electrode, because the free charge is shared by a plurality of pixel electrodes. Only when the free charge drifts near the pixel electrode, the induced charge on the corresponding pixel electrode will change rapidly. The induced charge on the single pixel electrode is almost entirely contributed by drifting of the charge in the vicinity of the pixel electrode. In the CZT detector with pixel anodes, the induced charges on the pixel anodes contributed by hole drifting are almost negligible, thus realizing the unipolar charge sensitivity technique and improving the energy spectrum resolution.
However, free charges will diffuse during drifting, and some of the charges will be collected by adjacent pixels, resulting in charge distribution problems. With the decrease in pixel size, the problem of charge distribution becomes more severe, making the energy spectrum resolution of one single pixel worse. For example, when a position at which a photon is incident is in the middle of two adjacent pixels, then charges induced by the incident photon will be collected by those two adjacent pixels, resulting in false signals in each of those two pixels. For another example, when a position at which a photon is incident is in the middle of four adjacent pixels, then charges induced by the incident photon will be collected by those four adjacent pixels. In actual situations, the position of the incident photon is uncertain, and a signal component contributed by respective pixels is also uncertain, so it is difficult to determine the accurate position of a ray.
Charge sharing may be corrected through signal compliance, but workload in circuit design is very large, and the efficiency of signal correction will not be very high. It is impossible to achieve real-time signal acquisition and analysis by data collection and then data processing.